Without limiting the scope of the invention, its background is described in connection with three-dimensional carbon-based structures.
The fast depletion of fossil energy and the associated adverse environmental impacts make it highly desirable to explore renewable-energy technologies. Carbonaceous materials with various morphologies and chemistries, such as carbon nanotubes1-3, bucky balls4, 5, graphene6-8, and thin graphite9-12, have emerged as key structures for energy storage and conversion devices13-17. Among them, thin graphite has received considerable interest as electrode supports owing to their high electric conductivity, excellent mechanical durability, and ultra-low mass density9, 18. However, it remains a challenge to rationally and efficiently synthesize carbonaceous materials into 3-D porous nanosuperstructures, which boast both high specific surface areas and fast ionic transports that significantly improve the performance of energy devices.
Previously, intensive research demonstrated the ultra-large specific surface area of graphene and its usage in energy devices, such as supercapacitors19, 20. Nevertheless, the assembly of graphene sheets is difficult to control, which could reduce the actual available surface areas and thus lower the device performance21. Recently, commercially available 3-D nickel foams were employed as catalysts for the synthesis of 3-D thin graphite22. Although this approach resolved the assembly problem of carbonaceous materials as electrodes for energy devices, the feature size of the as-obtained graphite resides at a scale of ˜100 μm. Complex chemical synthesis can produce porous carbon with pore sizes of a few nanometers23. Nevertheless, it remains extremely difficult to achieve 3-D carbonaceous nanostructures with multilevel porosity, which promises high surface areas and enhanced ionic transport24.